Geology Reference
In-Depth Information
Some aids for understanding joint origin on the basis of their geome-
trical expression, as seen in stereographic projections, are set out
below. These are often very helpful for interpreting geological history,
but as Price & Cosgrove (1990) put it:
'
be warned
-
many fractures
resist all attempts at interpretation
'
.
Many joint sets are orthogonal; two sets occur perpendicular to one
another and perpendicular to some planar fabric such as bedding,
schistosity, or
flow banding in an igneous pluton. Examples of such
joint sets in sandstone and granite are presented in
Figures 3.32
and
3.33.
The formation history can be quite complex, with one set being
formed initially, the second following stress relief due to the develop-
ment of the
first set or perhaps a general stress reversal, as discussed by
Rives et al. (1994). Interpretation may need detailed study of cross-
cutting relationships. For the practising engineering geologist, the
important thing is that this joint arrangement is very common in a
variety of rock types and this can aid in interpretation of sets from
field
Figure 3.32
Orthogonal
fractures in
sandstone, Table
Mountain, Cape
Town, South
Africa.
